Wallboard is a gypsum product widely used in building construction, including walls and partitions. Wallboard is typically manufactured by formulating a gypsum slurry in which calcined gypsum (calcium sulfate hemihydrate) is mixed with water and other ingredients. The gypsum slurry is then dispensed between two cover sheets of paper on a wallboard production line. The gypsum slurry is then allowed to set and forms a gypsum core between cover sheets. Processes for manufacturing wallboard are described in U.S. Pat. Nos. 6,494,609 and 6,986,812, both assigned to the United States Gypsum Company.
A wallboard panel has two long surfaces, one of the long surfaces is referred to as a back side and the other long surface is referred to as a face side. After installation is completed, the face side of the wallboard panel can be decorated, for example by painting and/or applying wallpaper. As described in U.S. Pat. No. 4,959,272, assigned to the United States Gypsum Company, two different types of paper are used as cover sheets for a wallboard panel. The first type of paper is used as a paper cover sheet which is used on the back side of the wallboard. This paper is formed from recycled wastepaper having many contaminants and it has a dark color.
The second type of paper is used on the face side of the wallboard. This is the side that will be decorated after installation. This second type of paper is typically a multi-ply sheet with 2 to 7 filler plies comprised of clean waste paper furnishes such as Kraft and boxboard cuttings of a hard stock nature and 1 to 2 top liner plies consisting of mainly flyleaf shavings and newspapers of a soft, typically mechanical pulp quality used to provide a light, manila colored smooth appearance on the face surface of the gypsum wallboard. The flyleaf shavings waste paper grade can be described as baled trim of magazines, catalogs and similar printed and unprinted material. It contains mechanical and chemical pulp fibers, but may also contain as much as 10-40 percent of clays and pigments from fillers and/or coating overlays.
As described in U.S. Pat. No. 5,945,198 assigned to the United States Gypsum Company, in order to provide a light, manila colored, smooth appearance on the exposed or decorated side, 1 to 2 top liner plies are included, covering the filler plies. This product is known as Manila or gypsum wallboard face paper. The top liner plies of Manila (manila face plies) consist mainly of soft stock furnishes such as flyleaf shavings and newspaper. A light-colored finish is desirable for the face paper because it may be covered with relatively few coats of paint without the risk of the underlying color “bleeding” through the paint. The flyleaf shavings can be described as baled trim of magazines, catalogs and similar printed and unprinted material. Containing primarily mechanical and chemical pulp fibers, but may also contain as much as 10-40 percent of clays and pigments from fillers and/or coating overlays.
Furnish components of the 1 to 2 top liner plies which are used to provide a suitable light colored, smooth face appearance are generally formed of 70-80% by weight of flyleaf furnish, old magazine and trim, and 30-20% waste newsprint. These top liner plies are inherently weak in strength. The high proportions of inert clays and fillers contained in the flyleaf shavings stock constituent further subtract from the strength of the liner plies. Moreover, the soft stocks and clays contained in the manila top liner plies are noted for poor dewatering, slow drainage, and drying.
Various attempts have been made to strengthen plies and especially top liner plies in a manila paper cover sheet. U.S. Pat. No. 4,959,272 assigned to the United States Gypsum Company, provides a paper cover sheet for gypsum wallboard which does not require the use of top liner plies. Instead, a coating composed of a light colored mineral filler and a binder is applied to an exposed surface of the cover sheet. Another method is provided by U.S. Pat. No. 5,945,198 assigned to the United States Gypsum Company, which provides a soy protein coating which can be applied over a paper sheet. However, applying various coatings over a paper cover sheet may increase the time needed for paper production. It may also affect painting and other decorative approaches.
Wallboard must meet ASTM standard requirements for compressive stress, flexural strength and other requirements. This presents a unique challenge with respect to paper that can be used in cover sheets for wallboard panels. A multi-ply manila paper that meets the ASTM requirements in lightweight wallboard is heavier than typical wallboard paper. However, decreasing the weight of paper may render this paper no longer suitable as a cover sheet because the paper no longer meets the requirements for strength and flexibility. Thus, there still exists the need for improved formulations for wallboard paper.